Advances in micro-scale and nano-scale integration have resulted in a new class of miniaturized electronic systems, such as smart dust sensors, wireless sensor nodes, and biomedical implants that enable new application domains. Despite constraints on size and hence battery capacity, these systems are often required to operate for several months or even years without the need for battery replacement. This is often due to the expense and/or infeasibility of frequent battery replacement. Environmental energy harvesting has emerged as an option to alleviate the energy supply challenge in these systems and to improve battery lifetime. Energy harvesting provides self-powered system operation.
Environmental energy harvesting has been explored and applied at the macro-scale in the context of large systems such as solar farms, windmills, and hydro-generators. However, a micro-scale energy harvesting subsystem for miniaturized electronic device involves different challenges. For example, the form-factor constraint in these systems mandates the use of small or miniature energy transducers. As a result, the output voltage of the transducer is typically low, such as less than 1V. For example, miniature single junction photovoltaic (PV) cells and thermo-electric generators (TEG) typically produce voltages in the range of 0.2V to 0.6V. Other energy sources, such as micro-fuel cells, also produce low voltages. Further, the maximum power output of micro-scale transducers is small, often only a few milliwatts. As such, the harvesting subsystem is designed to extract as much power as possible from the transducer and to transfer the extracted power to the electronic system. Multiple energy transducer modules may be connected in series or in parallel to generate higher output voltage. However, such a stacked arrangement is limited due to size, cost, and packaging considerations.
One component of a micro-scale energy harvesting subsystem is a power converter that boosts the output voltage of the energy transducer to a suitable level to enable energy storage in an energy buffer, such as a rechargeable battery, an ultra-capacitor, or another suitable storage device. The power converter is implemented using an inductive boost converter or a charge pump. Boost converters require an external bulky inductor, leading to an increase in system cost and size. Charge pumps include capacitors and metal oxide semiconductor (MOS) switches configured in a single- or multi-stage linear topology. The charge transfer capability of linear charge pumps is subject to degradation when used with ultra-low voltage energy transducers.
In one illustrated embodiment of the present disclosure, an energy harvesting system is provided for a micro-scale electronic device. The system includes an energy transducer configured to produce electrical energy. The energy transducer has an output configured to supply a first voltage. The system also includes an energy storage device configured to store electrical energy, and a power converter configured to transfer electrical energy from the transducer to the energy storage device. The power converter includes a charge pump coupled between the energy transducer and the energy storage device and a control unit configured to control operation of the charge pump. The charge pump includes a first stage, a second stage, and a third stage. Each of the first and second stages have an output and at least one input coupled to the output of the transducer so that the transducer supplies the first voltage to the at least one input of the first and second stages. The first and second stages of the charge pump provide second and third voltages at their outputs, respectively, the second and third voltages being greater than the first voltage. The outputs of the first and second stages are coupled to first and second inputs of the third stage of the charge pump and an output of the third stage supplies a fourth voltage to the energy storage device. The fourth voltage is greater than the second and third voltages.
In another illustrated embodiment of the present disclosure, a power converter is provided for an energy harvesting system of a micro-scale electronic device. The power converter is configured to transfer electrical energy from an energy transducer to an energy storage device. The power converter includes a control unit configured to provide at least one clocking signal, and a charge pump coupled to the control unit. The charge pump includes a first stage, a second stage, and a third stage. The first stage of the charge pump includes first and second inputs, the first input of the first stage being coupled to a first plate of a first capacitor through a first switch, and the second input of the first stage being coupled to a second, opposite plate of the first capacitor through a second switch, the first plate of the first capacitor also being coupled to a third switch which is also coupled to an output of the first stage. The second stage of the charge pump includes first and second inputs, the first input of the second stage being coupled to a first plate of a second capacitor through a fourth switch, and the second input of the second stage being coupled to a second, opposite plate of the second capacitor through a fifth switch, the first plate of the second capacitor also being coupled to a sixth switch which is also coupled to an output of the second stage. The third stage of the charge pump includes first and second inputs, the first input of the third stage being coupled to a first plate of a third capacitor, and the second input of the third stage being coupled to a second, opposite plate of the third capacitor, the first plate of the third capacitor also being coupled to a seventh switch which is also coupled to an output of the third stage. An output from the energy transducer is coupled to the first and second inputs of the first and second stages of the charge pump and the outputs of the first and second stages are coupled to the first and second inputs, respectively, of the third stage of the charge pump. An output of the third stage is coupled to the energy storage device. The at least one clocking signal from the control unit is coupled to the first, second third, fourth, fifth, and sixth switches to control operation of the charge pump so that voltages at the outputs of the first and second stages are greater than the voltage supplied by the energy transducer and a voltage at the output of the third stage is greater than the voltages at the outputs of the first and second stages.
In one illustrated embodiment, the second plate of the first capacitor is coupled through an eighth switch to ground, the second plate of the second capacitor is coupled through a ninth switch to ground, and the second plate of the third capacitor is coupled through a tenth switch to ground. In an illustrated embodiment, the control unit generates first and second non-overlapping clocking signals. The first clocking signal is supplied to the first, fifth, sixth, and eighth switches, and the second clocking signal being supplied to the second, third, fourth, seventh, ninth, and tenth switches to control operation of the charge pump.
In yet another illustrated embodiment of the present disclosure, a method for supplying power from an energy transducer to an energy storage device includes providing a charge pump including a first stage, a second stage, and a third stage; supplying a voltage source from the energy transducer to at least one input of the first and second stages of the charge pump; and increasing the voltage received from the voltage source in the first and second stages of the charge pump in parallel. The method also includes supplying output voltages from the first and second stages to first and second inputs of the third stage of the charge pump; increasing the voltage in the third stage compared to the output voltages of the first and second stages; and supplying the increased voltage from the third stage to the energy storage device.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.